At a Glance

Why Get Tested?

When to Get Tested?

When you have no signs or symptoms of an active infection, but your health care provider wants to determine if you are a MRSA carrier

Sample Required?

Swab of the nose; occasionally, swab of wound infection site or skin lesion

Test Preparation Needed?

None

The Test Sample

What is being tested?

Methicillin resistant Staphylococcus aureus (MRSA) are strains of Staphylococcus aureus, or "staph," bacteria that are resistant to the antibiotic methicillin as well as to related "beta lactam" antibiotics such as oxacillin, penicillin, amoxicillin, and cephalosporins that are used to treat ordinary staph infections. These tests detect the presence of MRSA in a patient's sample.

Staphylococcus aureus frequently colonizes the human skin and is present in the nose of about 25-30% of U.S. adults. It does not usually cause harm or symptoms. However, if there is a break in someone's skin from a wound or surgery, or if someone's immune system is weakened, then colonizing S. aureus can cause an infection.

Widespread use of antibiotics over the past several decades have led to the development of antibiotic-resistant strains of S. aureus. Infection with these strains can cause skin infections and potentially more severe, life-threatening infections such as pneumonia, bloodstream infections, or infections at a surgical site. However, according to the Centers for Disease Control and Prevention (CDC), less than 2% of the population carries the type of staph bacteria known as MRSA. [For more on this, see the article Staph Wound Infections and Methicillin Resistant Staphylococcus aureus.]

MRSA can be spread in health care settings and in the community. Contaminated hands, medical equipment, and surfaces in places such as hospitals, clinics, or nursing homes allow the spread of MRSA from colonized or infected patients. In the community, MRSA is usually spread by sharing personal care items, participating in contact sports, or living or working in a place where there is close contact with others – anything that allows for skin-to-skin contact.

Most hospitals have instituted measures to attempt to eradicate MRSA and to control the spread of MRSA from person to person. Nine U.S. states mandate MRSA screening of all patients in the intensive care unit (ICU) of a hospital. Those who test positive for MRSA may be isolated to prevent the spread to others and/or decolonized with a nasal antibiotic ointment and daily cleansing with special antimicrobial wipes. A study comparing prevention strategies for bloodstream infections found, however, that decolonizing ICU patients who are MRSA-positive based on screening tests was not as effective at reducing these infections as decolonizing all patients admitted to the ICU. As a result, hospital policies may change in the future and less MRSA screening may be performed in these settings. However, the state regulatory requirement for MRSA screening would have to be changed before hospitals in those states with the mandate could adopt a "no MRSA screening" policy.

How is the sample collected for testing?

A nasal swab is collected by rotating a swab inside each nostril. Occasionally, a swab of a wound infection site or skin lesion is collected.

Is any test preparation needed to ensure the quality of the sample?

The Test

How is it used?

A methicillin resistant Staphylococcus aureus (MRSA) screen is a test that looks solely for the presence of MRSA and no other pathogens. It is primarily used to identify the presence of MRSA in a colonized person. On a community level, screening may be used to help determine the source of an outbreak. On a national level, additional testing may inform clinicians and researchers about the unique genetic characteristics of the strains of MRSA circulating in the community or health care setting.

MRSA colonization may be determined by a bacterialculture or a molecular test that detects the DNA and resistance gene of MRSA. A nasal swab is collected from the nares (nostrils) of an asymptomatic person and cultured (put onto a special nutrient medium, incubated, and then examined for the growth of characteristic MRSA colonies). A swab may be collected from a wound site or skin lesion of a person who has been previously treated for a MRSA infection and cultured similarly. A screening culture identifies the absence or presence of MRSA and usually takes 1 to 2 days for a result.

Molecular tests for MRSA screening have the potential to detect nasal or wound carriage within hours, allowing for prompt treatment as necessary. The same specimen types are used in a molecular test, but the specimen is analyzed for the genetic markers to identify S. aureus and the mecA gene that confers resistance to methicillin, oxacillin, nafcillin, dicloxacillin, and other similar antibiotics.

Some hospitals have instituted measures to control the spread of MRSA by screening those patients they feel are at risk of being colonized with these resistant bacteria (a carrier) or all new admissions to the hospital. When an outbreak of MRSA is under investigation, screening of health care workers, family members, and close contacts may be performed to identify the source of the infection and to help devise a plan to contain these infections. In some settings, such as nursing homes, a large number of people may be screened to evaluate the spread of colonization in a specific population.

When is it ordered?

A MRSA screening test may be ordered when a health care provider, hospital, or health department needs to evaluate potential MRSA colonization in an individual, their family members, and/or a group of people in the community as the source of a MRSA infection.

Specific populations that have close physical contact, such as a sports team, residents of a nursing home, or health care workers, may be tested for MRSA carrier status when an increased number of infections occur within their close group.

Occasionally, a person who has been treated for MRSA infection or for MRSA colonization may be screened to determine whether MRSA are still present.

Is there anything else I should know?

Further testing may be performed on MRSA bacteria when they are isolated in a culture. These additional research tests can identify the type and subtype of S. aureus strains. Although the typing may not be used to determine treatment of the patient, it provides information to track the pattern of disease spread of the infection and characterize the toxins and other virulence factors present in the bacteria.

A study comparing prevention strategies for bloodstream infections found that decolonizing intensive care unit (ICU) patients who are MRSA-positive based on screening tests was not as effective at reducing these infections as administering a course of a nasal antibiotic and cleansing patients with special antimicrobial wipes for all patients admitted to the ICU. As a result, hospital policies may change in the future and less MRSA screening may be performed in these settings.

Article Sources

NOTE: This article is based on research that utilizes the sources cited here as well as the collective experience of the Lab Tests Online Editorial Review Board. This article is periodically reviewed by the Editorial Board and may be updated as a result of the review. Any new sources cited will be added to the list and distinguished from the original sources used.

Waknine, Y. (2006 March 31). Highlights from MMWR: Outbreaks of MRSA Infection in Newborns and More [On-line information]. Available online at http://www.medscape.com/viewarticle/528952 through http://www.medscape.com.

(2006). New Test Diagnoses MRSA in One Day. Medscape from Reuters Health Information [On-line information]. Available online at http://www.medscape.com/viewarticle/523635 through http://www.medscape.com.

Goossens, H. (2006). Antibiotic Resistance: Highlights of the 16th European Congress on Clinical Microbiology and Infectious Diseases. Medscape [On-line information]. Available online at http://www.medscape.com/viewarticle/530824 through http://www.medscape.com.

(2006). Community-Acquired MRSA a Leading Cause of Skin and Soft-Tissue Infections. Medscape from Reuters Health Information [On-line information]. Available online at http://www.medscape.com/viewarticle/524998 through http://www.medscape.com.

(Updated 2009 September 28). Recognize and Prevent MRSA Infections. Centers for Disease Control and Prevention [On-line information]. Available online at http://www.cdc.gov/Features/MRSAInfections/ through http://www.cdc.gov. Accessed March 2009.

(Updated 2009 June 18). FDA Clears First Quick Test For Drug-Resistant Staph Infections, Test Identifies MRSA Bacterium in Two Hours. U.S. [On-line information]. Available online at http://www.fda.gov/NewsEvents/Newsroom/PressAnnouncements/ucm161552.htm through http://www.fda.gov. Accessed March 2009.

(2010 January 21). Genetics Used to Track Transmission of MRSA Bacteria. MedlinePlus HealthDay [On-line information]. Available online at http://www.nlm.nih.gov/medlineplus/news/fullstory_94379.html through http://www.nlm.nih.gov. Accessed March 2009.

(Updated 2009 July 22) MRSA and the Workplace. National Institute for Occupational Safety and Health [On-line information]. Available online at http://www.cdc.gov/niosh/topics/mrsa/ through http://www.cdc.gov. Accessed March 2009.

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This article was last reviewed on August 6, 2013. | This article was last modified on May 13, 2014.

The review date indicates when the article was last reviewed from beginning to end to ensure that it reflects the most current science. A review may not require any modifications to the article, so the two dates may not always agree.

The modified date indicates that one or more changes were made to the article. Such changes may or may not result from a full review of the article, so the two dates may not always agree.